234 



however, of the mode in which the air contributes 

 to sustain animal heat, was attempted, till after the 

 great discovery of latent heat by Dr Black. That 

 excellent philosopher having already proved, that the 

 change effected in the air by respiration, consisted 

 in the formation of carbonic acid (116.), similar to 

 what happens in many examples of combustion, as- 

 cribed the production of animal heat to the decom- 

 position of the air in the lungs, by which its latent 

 heat was rendered sensible, in the same manner as 

 it is given out in combustion. The blood, in its 

 passage through the lungs, had, he conceived, its 

 temperature by thrs means raised ; and thus was 

 rendered capable of communicating heat to all parts 

 of the body, in the course of its circulation through 

 the system. To this it was objected by Dr Cullen, 

 that, if true, the temperature of the body ought to 

 be greatest in the lungs, and to diminish gradually 

 as the distance from the lungs increases, which is 

 not according to fact. This difficulty was removed 

 by the ingenuity of Dr Crawford, who, by a happy 

 extension of Dr Black's doctrine, maintained, that 

 the heat, liberated by the decomposition of the air 

 in the lungs, passed into the blood, and existed in 

 that fluid in the form of latent, or, what is now term- 

 ed, specific heat, in consequence of which its tempe- 

 rature was not raised ; and that this heat, by other 

 chemical changes, was given out by the blood in a 

 sensible form during its circulation. 



190. In what manner, then, does the air, breath- 

 ed by the superior animals, give out its heat, to sup- 

 port that high degree of temperature above the sur- 

 rouna'ing medium, which they all possess ? We 



